"Under Bernie’s plan, Americans will benefit from the freedom and security that comes with finally separating health insurance from employment. That freedom would not only help the American people live happier, healthier and more fulfilling lives, but it would also promote innovation and entrepreneurship in every sector of the economy," Sanders explains on his website.

Called "Medicare for All," the bill is still a longshot, and not just because Republicans control both chambers of Congress and the White House. It does not yet have majority support among Democratic lawmakers and several other attempts at single-payer health care at the state level have failed to come to fruition.

Here's what you need to know about what a single payer plan actually entails.

There's Not Just One 'Single Payer' Proposal

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The idea behind single-payer insurance is simple: The government, either at the state or federal level, is the main provider of health insurance. Proposals are often referred to as "Medicare for All" because, in theory, it would be similar to the government program that currently offers insurance to everyone over the age of 65.

But while the idea is simple, the execution is not. Proposals differ on whether the single-payer program would be the only option for insurance or a fall-back for people who can't get insured at work, as well as how the current health care system would transition.

Democratic Sen. Brian Schatz (who has signed on to a co-sponsor for Sanders' bill) told Vox last month he was working on a bill that would enable people to buy into Medicaid, a separate government-run insurance program offered for people in poverty. Democratic Sen. Chris Murphy told Politico he is working on something similar.

“We’re not going to pass a single-payer health care bill any time in the next few years. And so we need to have a conversation about how we get there,” he said.

The Idea Has Faltered at the State Level So Far

In 2014, Sanders' home state of Vermont abandoned its plan to become the first state to offer a single-payer healthcare system, with then-Governor Peter Shumlin conceding that taxes would have to be raised too much. In California, a single-payer bill passed the Senate, but the Speaker of the Assembly decided it would remain in committee because it didn't adequately deal with implementation, including cost.

A Medicaid for All plan made it through the state legislature in Nevada, but Republican Gov. Brian Sandoval vetoed it, arguing that the bill "raises more questions than answers, while adding more uncertainty to an industry that needs less."

But Its Gaining Ground Among Voters and Lawmakers

Sanders also put forward a single-payer bill in 2013, but none of his colleagues joined him in the effort. Now, he has 15 co-sponsors, including high profile Senators with speculative presidential ambitions in 2020 such as Massachusetts' Elizabeth Warren and California's Kamala Harris. Key leaders, such as House Minority Leader Nancy Pelosi and Senate Minority Leader Chuck Schumer, do not back the bill.

But the increased level of support Sanders is receiving follows polls showing the idea is gaining popularity among the public. A 2014 poll by Gallup showed that 61% of Americans said they preferred a private health insurance system, while 43% wanted one run by the government, a gap of 26 percentage points. But a more recent poll from December showed that gap had narrowed to 10 percentage points, with 53% saying they preferred a privatized health insurance system, while 43% wanted one run by the government.

The Kaiser Foundation, which conducted its own tracking poll on the subject and found similar results, also noted that support for a government run system has increased among independents, from 42 percent support in 2008-2009 to 55 percent in 2017.